El Nino almost always produces good seasons in South America and tends to produce below average seasons in Australia/New Zealand. I cleverly went to NZ in 1982 and 1997, the 2 biggest El Nino years on record.
The current North American season
looks like a classic El Nino, but in fact it's mostly coincidence. The El Nino was always weak, and now it's dissipating.
I am spending a couple of days with Extremely Canadian at Whistler in March, and I am considering one of their Las Lenas trips in September.
The simple advice, which I think has been covered before, is that if you want a dedicated week of skiing every day, go to South America. Bigger lift-serviced terrain, much higher altitude with better snow preservation.
NZ is an outstanding tourist destination with many interesting things to do: friendly people, safe, easy to get around on your own with a car. Plan on a flexible schedule, skiing no more than half the days you're there. Given weather, reliability and the cost of getting there, I would advise against committing to a specific area.
I wrote an Inside Tracks article after my 1997 trip.
My own leaning toward South America has nothing to do with expected weather this year. I've been to NZ twice and I like to visit new places.